The present invention relates to spray drying technology applicable within a broad range of industries, e.g. the pharmaceutical, chemical and food industries.
The term spray drying is here used in a broad sense as meaning not only processes for transforming a solid dissolved or suspended in a liquid into a powdery, possibly agglomerated material, but also processes in which an essential purpose is to agglomerate a particulate material by spraying and drying a liquid thereon.
Within the area of spray drying substantial improvements have been developed and implemented within the last many decades. A standard textbook on spray drying technology is Masters, Keath: Spray Drying Handbook, 5th edition, (Longman Scientific and Technical 1991), incorporated herein by reference.
Modifications of the basical spray drying process which are of special relevance in connection with preferred embodiments of the present invention include the application of an internal stationary fluidized bed in the bottom portion of the spray drying chamber as well as the arrangement of a filter in the very spray drying chamber to retain particles in the chamber otherwise removed therefrom entrained in the stream of spent drying gas exhausted from the chamber.
A process and an apparatus utilizing a fluidized bed in the bottom of the spray drying chamber is described i.a. in U.S. Pat. No. 5,632,100 (Hansen). In the embodiments described in said US patent, various types of particle collecting equipment are used for treating not only the spent drying gases withdrawn from the drying chamber but also to treat particle-loaded gas streams resulting from after-treatment of the product particles recovered from the drying chamber in fluidized bed apparatuses or gravitational classifiers.
WO 97/14288 suggests incorporation of special, rigid gas filters into the very spray chamber whereby substantial advantages may be obtained, viz. improved conditions for particle agglomeration in the drying chamber and a reduction of the requirement for particle collecting equipment such as cyclones and filters, including electrostatic filters and fabric filters for treating the spent drying gases from the drying chamber.
Said WO 97/14288 discloses an embodiment in which a product recovered from a first stationary fluidized bed in the bottom of a spray drying chamber having internal, rigid gas filters is subjected to an after-treatment in an annular fluidized bed encircling said first stationary, fluidized bed and having a common ring-shaped partition wall therewith. The gas with entrained fine particles from said annular fluidized bed is introduced into the drying chamber through an annular horizontal slit in short distance above said first fluidized layer, and subsequently passes the gas filters when leaving the chamber together with spent drying gases, whereby said fine particles collect on the filter surfaces.
However, this embodiment has some drawbacks because the presence of said annular slit, which is placed in or just below the walls of the conical bottom portion of the drying chamber, interferes with the first fluidized bed by preventing the desired spouting function thereof. A spouting of material of the first fluidized bed up on the conical walls is essential to keep these free of deposited sticky material. The slit with the upward gas flow therethrough prevents proper return of the spouted material into the first fluidized bed, and involves a risk for relatively moist material dropping down into the annular fluidized bed.
Besides, the use of an annular fluidized bed encircling said first fluidized bed for the after-treatment involved some limitations as to the type of the after-treatment and the operational parameters and the controlling and adjustment possibilities thereof. Therefore it might be preferred to perform the after-treatment in a separate unit optimal for the relevant treatment, in which independently adjustable operational parameters can be applied.
Therefore, it is conventional to use one or more independent, separate apparatuses for the after-treatment.
Whether the after-treatment is a supplementary drying, a cooling, a classification, an agglomeration or a separation, a particle-loaded gas stream is formed, from which the particles must be separated to recover values therein or to avoid environmental pollution.
Therefore, the particle-loaded gas stream withdrawn from the separate after-treatment apparatus is in the prior art processes passed through collection means serving solely-the purpose of separating the particles from said gas stream. Also special measures had to be taken to incorporate the separated particles into the main product stream or for utilizing them in other applications.
These means for collecting the particles from the gas stream from the separate after-treatment apparatus and the means for handling of the collected particles increase the total plant costs and also increase the space requirements.
Thus it is an object of the present invention to provide a process and a plant, which in comparison with the closely related prior art are simplified by omitting the external collector means for collecting the particles from the gas from the separate after-treatment apparatus.
It has now turned out that it is possible to introduce fine particles entraining gas streams from after-treatment processes, e.g. in fluidized beds, classifiers etc., into the spray drying chamber and thereby utilize the integrated filter in said chamber to clean said gas streams and thus make auxiliary particle collecting means outside the drying chamber redundant. This can be done without disturbing the primary drying gas flow and the drying and agglomeration processes in the drying chamber, and also without decreasing the quality of the resulting product.
Thus, the invention deals with a process for spray drying a liquid to produce an agglomerated product by atomizing the liquid into a drying chamber to form droplets, introducing a first stream of drying gas downward into the upper part of said chamber to partially dry the droplets, introducing a second stream of drying and fluidizing gas upward in the bottom portion of the chamber through a perforated plate to maintain a fluidized layer of particles on said plate, withdrawing a stream of spent drying gas comprising gas from said first and said second stream of gas from the chamber through particle collecting filter means thereby retaining particles on the surface of said filter means, releasing said retained particles from the filter means, to allow their contacting other particles in the chamber for agglomeration and transfer into the fluidized layer, recovering particles from said fluidized layer and by an external pipe means transferring these to an external, separate after-treatment unit and subjecting said particles to at least one after-treatment selected from after-drying, cooling, classification including dust removal, agglomeration, coating and separation, involving gas contact in said unit, and withdrawing a stream of dust-loaded gas from said unit through second external pipe means. According to the invention this process is characterized in that said dust-loaded gas stream is by said second external pipe means conducted into the drying chamber, whereby the gas in this stream passes through the particle-collecting filter inside the chamber before leaving the chamber together with said spent drying gas.
The spray drying process may be of the kind in which the liquid being atomized contains a dissolved or otherwise dispersed solid, which after the drying constitutes the final product.
Alternatively, the process may be characterized in that a particulate solid is injected into the chamber for agglomeration promoting contact with the atomized liquid droplets or moist particles formed by partial drying thereof, and in that solids contained in the liquid show adhesive properties when dried and/or the liquid comprises a solvent evoking stickiness of one or more components of the particulate solid.
To perform the process of the invention certain requirements must be fulfilled by the equipment.
Consequently, the invention also deals with a plant for performing the above specified process, said plant having a drying chamber, an atomizing device in said chamber connected to a source for liquid to be spray dried, a drying gas disperser in the upper portion of the chamber, a perforated plate in the bottom portion of said chamber, a plenum below said plate for providing an upward flow of fluidized and drying gas through the perforations in said plate, sufficient for maintaining a layer of fluidized particles on said plate, a particle-collecting filter inside said chamber, means for withdrawing a stream of gas from said chamber through said filter, outlet means for recovering particles from a fluidized layer existing on said plate during the operation of the plant, and, connected to said outlet means, first external pipe means conducting the recovered particles to at least one external, separate after-treatment unit selected from an after-dryer, a cooler, a classifier, an agglomerator, a coater and a separator, which unit during the operation thereof emits a stream of gas entraining small particles
According to the invention in its broadest aspect, this plant is characterized in having second external pipe means for introducing said stream of gas with entrained fine particles emitted from the unit into said drying chamber at a location upstream of said filter.
Typically, said at least one external, separate after-treatment unit is a fluidized bed apparatus for drying and/or cooling the particles provided from the chamber through said first external pipe means. However, it may just as well be a counter-current/gravity classifier.
In preferred versions of the plant, the integrated filter is located in an upper portion of the spray drying chamber.
Most experiences have been obtained with drying chambers having a lower portion of downwardly decreasing horizontal cross-section area, the bottom part of which portion accommodates the perforated plate and the fluidized layer and the filter being located above the slanted wall of said lower portion.
The amount of fine particle-loaded gas produced in the after-treatment may be large, e.g. corresponding to up to 50% of the amount of gas introduced into the drying chamber as drying and fluidizing gas. To avoid that such a large amount of gas disturbs the flow pattern existing in the chamber, which flow pattern is essential for obtaining the desired operation and product qualities, it is preferred that the external second pipe means connecting the after-treatment unit and the drying chamber debouches in an upper portion of the latter on a level with the filter.
In such an embodiment the filter may comprise at least one essentially vertical member of which at least a part is between the atomizer and the location in the upper portion of the chamber for debouching of said pipe means for introducing the gas stream with entrained fine particles.
In a specific embodiment of this last-mentioned version of the plant the filter comprises a plurality of vertical cylinders encircling the atomizer device, the second pipe means for introducing the gas stream with entrained fine particles debouching at least at one location outside the one encircled by the cylinders and at a distance to the nearest of said cylinders not less than 1.5 times the diameter of this cylinder.
By this arrangement it is avoided that the fine particles from the external after-treatment unit contact the hot areas in the drying chamber. This is essential since said fine particles are due to their lower moisture content more at risk of being heat-damaged than the average particles present in the drying chamber.
This last-mentioned embodiment furthermore has the advantage that the risk for damage of the filters due to abrasion is minimized, by avoiding impingement by very fast particles on the filter surface. Furthermore, a uniform building-up of particles on the filter surface is obtained.
The versions of the plant described above have a stationary fluidized layer at the bottom of the drying chamber. However, it might be possible to achieve some of the advantages inherent in the invention by using a more conventional spray drying chamber without fluidized layer in the bottom, especially if a sufficient agglomeration of the fine particles introduced from the external after-treatment unit is ensured.
The invention and subject-matter of claims not dealt with above is explained below with reference to the drawings.